Darth Vader, Dark Lord of the Sith, was truly under the leash of Governer Tarkin. Why, because Vader-Unleashed would have destroyed any and all who dared to even disrupt his train of thought! He was a master of the dark-side and had completely destroyed all traces of Anakin after discovering his failing with Padme and his child(ren).

But he too was infected with a weakness. A good, that he had all but forgotten. He faced his own child. He faced Luke, The Son of Skywalker and knew that Anakin had not failed as utterly as he had been led to believe!

And Luke knew . . .
He understood, perhaps better than Vader himself, the conflict that his very existence had sparked inside Vader's iron heart.

Then, came the redemption of Vader, and the fulfillment of the prophesy!
Vader stood, witnessing his son in pain at the hands of the dark-side! He saw and felt the anguish of Luke and could hear the desperation in his calls for help!
He was being tortured, as Anakin's mother had been, and would soon end up dead! This would not be the failing of Anakin, but the failing of Darth Vader himself! He had achieved the power he needed. He could save his son!
And he did . . .

He destroyed the Emperor! Not to restore balance to the force, not to right any wrongs he had done, but to save his son from death! To save Luke Skywalker!
To save the memory of Anakin, Jedi Knight of The Republic . . .